Performance Estimation of UTC and Several UTC(k) Realizations During the Past 25 Years

This article analyzes data published by the Bureau International des Poids et Mesures (BIPM) over the past 25 years to assess the long-term performance of the Coordinated Universal Time (UTC) timescale and ten other physical realizations of UTC. We utilize a combination of the three-cornered hat method and dynamic Allan deviation estimation to provide a comprehensive evaluation of UTC performance over such an extended period of time. Furthermore, we present a forecast of UTC’s performance for the next ten years. We highlight key events based on data published by the time department of the BIPM during the period from January 1998 to December 2023. We determine that UTC instability, for averaging periods ( $\tau $ ) of five days and one year, has decreased by an order of magnitude in respective periods of 18.9 years and 16.9 years, respectively. If this improvement continues during the next decade, we estimate that the instability of UTC will decrease to $7.8 \times 10^{-17}$ for $\tau =5$ days and $1.1 \times 10^{-17}$ for $\tau =1$ year. We discuss the correlation between the development of primary and optical frequency standards, the improvement in time transfer methods, and the performance of the UTC(k) timescales.